In general, polypropylene resins (PP), acrylonitrile-butadiene-styrene resins (ABS), polyamide resins (PA6, PA66, etc.), polyester resins (PET, PBT, etc.), polycarbonate resins (PC) and the like are used as materials for resin molded/formed products. Although products produced from any of these resins are excellent in moldability/formability and mechanical strength, these resin products lead to an increased amount of garbage when being discarded. In addition, the resin products are hardly decomposed in the natural environment and, therefore, permanently remain in the ground after landfill disposal.
In recent years, biodegradable polyester resins have come into focus from the viewpoint of environmental preservation. Among theses resins, polylactic acids, polyethylene succinates, polybutylene succinates and the like are less expensive because of its mass-productivity, and very useful. Particularly, the polylactic acids can be industrially produced from plants such as corn and sweet potato. Even if the polylactic acids are incinerated, the polylactic acids are carbon-neutral in consideration of carbon dioxide absorbed during the growth of these plants. Therefore, the polylactic acids exert a lower load to the global environment.
Where the polylactic acids are sufficiently crystallized, the polylactic acids are improved in heat resistance and, therefore, find a wide variety of applications. However, the polylactic acids per se are very slowly crystallized. Typical approaches to improvement of the crystallization speed are to add any of various crystal nucleus agents to the polylactic acid and to crosslink the polylactic acid.
A specific method for the addition of the crystal nucleus agent for the promotion of the crystallization of the polylactic acid is to add a carboxylate or a carboxylamide having a specific molecular structure to the polylactic acid as disclosed in WO2006/137397. Further, JP2006-328163A discloses addition of tricyclohexyltrimesamide to the polylactic acid. JP2003-226801A discloses addition of ethylene bis(12-hydroxystearamide) to the polylactic acid.
An exemplary method for the crosslinking of the polylactic acid for the promotion of the crystallization of the polylactic acid is to blend a methacrylate compound in the polylactic acid as disclosed, for example, in JP2003-128901A. JP2002-3709A discloses blending of an isocyanate compound in the polylactic acid.
However, even if the crystal nucleus agent is added to the polylactic acid or the polylactic acid is crosslinked, a mold temperature should be set at 90° C. to 120° C. to provide a practically sufficient crystallization speed when a product is to be injection-molded from the polylactic acid with the use of a mold.
Therefore, even if the polylactic acid is sufficiently crystallized, the molded product has a lower rigidity when being demolded by ejection. To cope with this, a molding cycle should be prolonged. As a result, the polylactic acid molded product is produced at a lower productivity than the prior art resin molded products. This significantly hampers extensive applications of the polylactic acid resins and the products molded from the polylactic acid resins.
Where the mold temperature is set at 90° C. to 120° C., a temperature difference between the mold temperature and the room temperature is increased, resulting in a greater contraction ratio of the polylactic acid molded product. Therefore, the polylactic acid molded product has limitation in applications which require a higher dimensional accuracy. The higher contraction ratio of the molded product makes it impossible to use a mold designed for existing resins, so that the mold cannot be shared.
On the other hand, use of a polylactic acid having a higher optical purity ensures the promotion of the crystallization, thereby improving the heat resistance. As stated, for example, in JP2007-051274A, it is preferred that L-lactic acid is present in a proportion of not less than 95 mol % or D-lactic acid is present in a proportion of not less than 95 mol % in the total lactic acid component for the improvement of the heat resistance.
A polylactic acid resin having an L-isomer content of 98 to 99 mol % is typically used, for example, for the injection molding of the polylactic acid resin. However, even if the polylactic acid resin having an L-isomer content falling within this range is used in combination with the crystal nucleus agent, the mold temperature should be set at 90° C. to 120° C. for the molding. That is, if the mold temperature is lower than this temperature range, the polylactic acid crystallization speed is significantly reduced, making it impossible to produce a product of the polylactic acid having a sufficient heat resistance in a practical injection molding cycle.